With the advent of probiotics sold as health supplements, it has become common to refer to bacteria in the intestinal tract as "good" bacteria or "bad" bacteria. The average person hosts around 100 trillion bacteria in the gut, according to the Harvard Medical School Family Health Guide. "Good" bacteria perform a number of necessary functions in the intestinal tract. Some "bad" bacteria cause no problems as long as their numbers remain low but can make you sick if they multiply or travel to other parts of your body.
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Defeating Bad Bacteria
Good bacteria help keep bad bacteria from multiplying and causing illness in several ways. They use up nutrients so that bad bacteria don't have access to them. Lactobacilli, found mostly in the small intestine, and Bifidobacteria, found primarily in the large intestine, maintain an acidic environment in the intestines by producing lactic acid, hydrogen peroxide and acetic acid. An acidic environment discourages the growth of bad bacteria. Good bacteria also strengthen the mucosal tissue that lines the inside of the intestines, making it harder for toxins to gain a toehold and take up residence.
Food digestion depends on the actions of good bacteria, which break down complex sugars, proteins and fats so your body can absorb their energy. Lactobacilli, the dominant microorganisms found in the small intestine, break down proteins as well as lactose, which is found in dairy products, and cholesterol for absorption.
Some bacteria also synthesize vitamins. Lactobacillus acidophilus manufactures vitamin K. Lactic acid bacteria produce some B-complex vitamins, while enteric bacteria produce vitamin B-12.
Aiding the Immune System
Good bacteria play a powerful role in supporting the immune system. One type of Lactobacillus, L. caseii, produces bacteriocins, compounds that restrict the growth of harmful bacteria in the small intestine. Good bacteria increase white blood cells called T-cells, which aid in the immune system's response to harmful bacteria. A review of available studies conducted by the Veterans Administration Puget Sound Health Care System and published in the April 2006 issue of "The American Journal of Gastroenterology" found that taking probiotics, which consist of "good" bacteria, can shorten the duration of diarrhea after antibiotic administration and in Clostridium difficile infections.